1. Technical Field
The present invention relates to surgically correcting a deformed and/or degenerated spinal column, and particularly, relates to a connector for attaching a corrective device to a vertebra of a spinal column.
2. Description of the Prior Art
Connectors for attaching corrective devices to vertebrae of a spinal column are known. U.S. Pat. No. 4,648,388 discloses clamps for attaching a bendable rod to vertebrae. One type of clamp includes a clamping portion with an opening for receiving a portion of the rod. The opening has a substantially constant diameter along its longitudinal extent and the diameter of the opening is slightly larger than the outer diameter of the rod. A pair of legs extend from the clamping portion of the clamp and are located adjacent one another. Each of the legs has an opening extending transversely to the opening in the clamping portion. A fastener is received through the openings in the legs to press the legs together and to contract the clamping portion around the rod to grip the rod.
Another type of clamp of U.S. Pat. No. 4,648,388 includes a pair of substantially identical clamp halves. Each clamp half has an arcuate clamping portion with a leg extending therefrom. The arcuate clamping portions of the clamp halves define an opening for receiving the rod. In either type of clamp, once the legs engage, further tightening of the fastener applies very little additional force to the clamping portion to increase the clamping force on the rod.
If a bending moment is applied to the rod which is received in either type of clamp of U.S. Pat. No. 4,648,388, the rod tends to pivot about a point of contact within the clamping portion. If a straight portion of the rod is received in the clamping portion, the rod contacts the clamping portion at diametrically and axially opposite end portions of the clamping portion during the pivoting of the rod. The axial distance between the points of contact of the rod and clamping portion defines a moment arm of a length approximately equal to the longitudinal extent of the clamping portion. A force is applied by the fastener to the legs of the clamp through the moment arm to resist opening of the clamping portion when the rod pivots. If a bent portion of the rod is received in the clamp and contacts the clamp at an end portion and at a point intermediate the axial ends of the clamp, the moment arm is less than the longitudinal extent of the clamp. Thus, a larger force is required to be applied by the fastener to resist opening of the clamping portion due to the same magnitude bending moment in the rod because the force is applied through a shorter moment arm.